After the war, Irwin returned to the United States and became commander of V Corps in 1946 and director of the Military Intelligence Division in 1948. He finished his military career as the commander of U.S. Army forces in Austria from 1950 to 1952 when he retired due to medical problems.

Irwin married in 1921 to Helen (Hall) Irwin and together they had one son, Francis LeRoy. After Helen died in 1937, Irwin remarried in 1941 to Clare (Moran) Irwin. His second marriage also produced a son.